The process of silver halide emulsion precipitation is a complex physiochemical phenomenon that is characterized by two competing kinetic processes: a) the kinetics of precipitation; and b) the kinetics of mixing. The kinetics of precipitation may be described by a complex sequence of competitive and consecutive chemical reactions, while the kinetics of mixing is determined by the physical characteristics of the mixer and the hydrodynamics of the medium that is being mixed. In the case of silver halide emulsion precipitation, the medium is a colloidal suspension of water, gelatin and silver halide particles.
In a typical silver halide emulsion precipitation process, aqueous solutions of silver nitrate and alkali halide (NaBr, KI, NaCl, etc.) are introduced into the reactor using a mechanical pump and mixed rapidly by a rotary agitator. The physical characteristics of the silver halide emulsion that results from the precipitation process are determined by the details of the interaction between the physical (mixing) and the chemical (precipitation) processes. The inherent chemical kinetics of the precipitation reaction are extremely rapid, relative to the kinetics of mixing. The chemical reactions that participate in the precipitation process may be regarded as instantaneous phenomena, relative to the sluggish transport of the species participating in the physical process.
The kinetics of mixing can be described by two different rate processes; a) the kinetics of micromixing, which determine the time required to eliminate the microscopic inhomogeneities (by molecular mixing) between regions that have dimensions on the order of the smallest hydrodynamic turbulent length scale in the reactor, and b) the kinetics of macromixing which determine the time required to achieve a homogenous (macroscopic) distribution of the species introduced into the reactor. In the case of silver halide emulsion precipitation, the kinetics of micromixing determine the chemical identity of the precursors to the precipitation process (nucleation and growth), while the kinetics of macromixing are responsible for the homogeneity in the distribution of these precursor species in the reactor. To summarize, both macromixing and micromixing are important in achieving a controlled precipitation of silver halide emulsions.
Generally, during the precipitation of silver halide emulsions, both micromixing and macromixing are achieved using a single rotary agitator. Because the kinetics of micromixing and macromixing are very different, this approach will not provide optimal micromixing and optimal macromixing in the reactor. That is, a single device that is designed to carry out both tasks simultaneously, will necessarily perform one or both of the tasks in less than an optimum manner.
U.S. Pat. No. 4,289,733 addresses this problem by disposing a polygonal mixing chamber within a reaction vessel and using two independently controlled, concentric rotary agitators within the mixing chamber. One of the rotary agitators is used for optimum micromixing and fresh reactants solutions are introduced in close vicinity of this agitator. The other agitator is located slightly above the first agitator and is used for optimum macromixing. One notable feature of this configuration is that there is a high circulation of reactor vessel contents through the reactant introduction region. On one hand, the introduction of reactants to the region of high turbulence is desirable but on the other hand, in many situations, the high circulation of emulsion crystals through that region may prove to be disadvantageous as emulsion crystals may be exposed to regions of high concentration of unreacted reactants and also to high supersaturation levels. Exposure of emulsion crystals to regions of high concentration of unreacted silver salt solution can lead to unintended formation of fog centers. Similarly, exposure to regions of very high supersaturation can lead to undesirable morphological changes such as thickness growth of tabular crystals.
The present invention provides a method and apparatus to improve on the prior art problems. This is done by locating reaction introduction points farther from the macromixing agitator and yet generating efficient micromixing of the reactants by a non-rotary agitation means.